Where galaxies collide: star-forming regions of NGC 92

By Matthew Owens

A Chilean-led team of astronomers has shed new light on the evolution of galaxies by observing young star formations in outer regions of the interacting galaxy NGC 92.

The unassumingly titled NGC 92 galaxy isn’t a close one. On the contrary, it’s quite far away, 160 million light years away to be precise, or roughly 940 quintillion miles. That’s a lot of noughts. NGC 92, the largest galaxy in what’s known as Robert’s quartet, is fairly unusual in that it is the proud owner of a tidal tail – an elongated stretch of interstellar gas formed by gravitational interaction with nearby galaxies. Astronomers know there are galaxy-galaxy interactions in this group because there’s a giant bridge-of-gas from NGC 92 to one of its neighbours (NGC 88). To understand the properties of interacting galaxies further we must travel back to the beginning of time.

About 14 billion years ago there was a day without a yesterday. Within a trillionth of a trillionth of a trillionth of a second, the universe expanded faster than the speed of light turning into a primordial soup of quark-gluon plasma; building blocks for protons and neutrons. Mostly hydrogen and helium was produced then, while metals (basically anything other than hydrogen and helium to astronomers) had to wait some 400 million years for the first stars to be born. These heavier metallic elements, such as the carbon and oxygen that we are made from, were forged in the cores of stars through the magical transformation of stellar nucleosynthesis.

In this sense, NGC 92 has a mixture of gases coming from different regions of different galaxies, which is amazing.

Astronomers can use the metallicity of a region, the ratio between hydrogen and helium and everything else, to understand more about the universe. Because the early universe was comprised mostly of hydrogen and helium, stars with low metallicity are much older than those with high metallicity. Crucially, the variance in metallicity can be used to plot the distribution, or gradient, in a galaxy. In normal galaxies astronomers expect to find a higher content of metals in the central regions relative to the periphery. However, as lead author Dr Sergio Torres from Universidad de La Serena explains, there is something special about interacting galaxies like NGC 92. They are as he says, “perfect laboratories to study galaxy evolution.” “In interacting galaxies the scenario is different, given that the interaction process can mix metals coming from the internal and external regions of the galaxies. Interestingly, we found that this system [NGC 92] displays a flat metal distribution, that means we detect the same amount of metals in the inner region and in the tidal structure.”

This flat distribution, as opposed to that in normal galaxies, is probably explained by an influx of non-metal rich gas at the centre, spewn forth from companion galaxies that enhanced the stellar mass. Furthermore, in the study the metallicity detected in the outer tidal tail region was extremely high (similar to the sun) when compared with non-interacting galaxies, suggesting that the star regions were born from a pre-enriched material ejected from the centre of NGC 92.

“In this sense, NGC 92 has a mixture of gases coming from different regions of different galaxies, which is amazing.”, said Dr Torres.

The star formation regions themselves could theoretically have been ejected from the interacting galaxies but this is not what the team found. The discovered regions were much younger than the galaxy itself and so were almost certainly formed in situ. “This fact is very interesting, given that we can study the physical properties of regions that were formed just a couple of million years ago”, said Dr Torres. In interacting galaxies, about 10% of star formation occurs in the tail and about 1% of all known stars occur in tidal tails.

But how certain can we be that the metallicity really came from surrounding galaxies? Torres admits that, “We can not discard that the detected metallicities in the tidal tidal were produced by a continuous star formation event in that structure. The next step in this study is to enlarge the sample of galaxies that has tidal tails, in order to determine if this finding is typical or not. Currently we are doing that with some graduate students here in Universidad de La Serena.”